The invention relates generally to improvement of the characteristics of polymers.
It is known to change the properties of high polymers by grafting monomeric substances onto the same. The grafting of the monomeric substances may be accomplished using radiation-chemical techniques. Thus, it is possible to initially irradiate the high polymer with ionizing radiation, for instance, with electron beams generated by a Van de Graaff generator, and to subsequently bring the high polymer into contact with a monomeric substance, which latter may be in the form of a liquid. This procedure is the so-called "pre-irradiation" method. The radiation used for irradiating the high polymer is referred to as ionizing radiation since it causes ions or radicals to be formed in the high polymer. It is also possible to first contact the high polymer with a monomeric substance, which latter then penetrates into the high polymer, and to thereafter irradiate the high polymer, which has been thus "loaded" with the monomeric substance, with ionizing radiation. The latter procedure is the so-called "simultaneous" method. In the latter method, there often occurs, in addition to the desired grafting reaction, an undesired homopolymerization of the monomeric substance.
Depending upon the monomer selected for grafting, different modifications of the properties of the high polymer may be obtained.
To illustrate the effects which occur, consider, for instance, a textile web composed of polyamide fibers and having the following structure: ##STR1## If the web is subjected to the effects of ionizing radiation, favorably irradiation with electrons, then there are primarily formed macro-radicals such as, for example, the following: ##STR2## These macro-radicals are capable of causing the polymerization of a monomeric substance such as, for example, acrylamide (CH=CHCONH.sub.2). The monomeric substance grafts onto the high polymer as a side chain and, if the high polymer is represented by - A - A - A - A - A - and the monomeric substance is represented by B, the effects which occur may be represented schematically as follows: ##STR3##
If a fabric composed of polyamide fibers is used as the high polymer and acrylamide is used as the monomeric substance, then one obtains a fabric of polyamide fibers which is characterized by exhibiting an increased capability for taking up moisture, that is, characterized by better physiological properties when used as a garment. On the other hand, when a textile web composed of polyester fibers is utilized as the high polymer and acrylic acid (CH.sub.2 =CHCOOH) is utilized as the monomeric substance, then there is obtained a textile web which can be colored using basic dyes. These two examples illustrate some of the modifications in properties which are achievable by grafting a monomeric substance to a high polymer. The methods outlined above provide a homogeneous or uniform modification of the properties of high polymers.
The types of radiation which may be used to obtain the radiation-chemical initiated grafting reaction include all those which are able to cause ionization or the formation of radicals. Of particular applicability here are electron beams, gamma rays and x-rays.
A disadvantage associated with the known methods for grafting monomeric substances onto high polymers and, in particular, onto textile webs, resides in that the textile character, that is, the appearance, of the web is not changed.
Another process is known wherein the shrinkage, that is, the decrease in size, of the web which occurs as a result of grafting is put to use for the purpose of obtaining structural or textural effects in the web. In other words, the shrinkage which occurs is used to achieve volumetric distortions of the web. Here, the free radicals requried for effecting graft polymerization are not produced homogeneously or uniformly over the entire surface of the web as in the methods discussed above but, rather, are produced in localized regions of the web. It is possible, for the purpose of local production of the radicals, to use templates which are interposed between the radiation source and the web so that radicals are produced only in locations of the web which correspond to the apertures in the templates, that is, only in locations of the web which can be reached by the radiation. On the other hand, it is also possible to homogeneously or uniformly irradiate the web with ionizing radiation and to subsequently locally destroy the free radicals in selected regions of the surface of the web. The localized destruction of the free radicals may, for instance, be carried out using heated, profiled rollers about which the irradiated web is conveyed.
When a web which contains free radicals in localized regions thereof is contacted with a monomeric substance, shrinkage occurs locally in these regions by virtue of the grafting reaction and the localized shrinkage results in volumetric distortions of the web. The volumetric distortions of the web lead to structural effects, that is, to ornamental patterns, which latter may be combined with color patterns. Moreover, the moisture sorption characteristics of the web may be enhanced.
The latter method, which results in a so-called partial modification, has a very disadvantageous aspect associated with it. This resides in that it is not possible to achieve those effects which require a homogeneous or uniform grafting of the monomeric substance. Examples of such effects include the obtention of good anti-static properties, which require the presence of a continuous and conductive "film" and the obtention of hydrophobic properties, which require the presence of a continuous and hydrophobic "film".